1. Field of the Invention
The present invention relates to the design of integrated circuits, and more specifically to a technique which minimizes the number of masks to be changed when changing existing connectivity in an integrated circuit.
2. Related Art
An integrated circuit contains circuit elements such as transistors, resistors, capacitors etc., connected according to a circuit design. In general, the circuit elements are fabricated in a base layer, and a metal layer above the base layer provides interface to the terminals of the circuit elements. Thus, with reference to FIG. 1, circuit elements are fabricated in base layer 110, and metal layer 120 provides via for interface to the terminals of the circuit elements.
Connectivity between the circuit elements is attained by providing a metal path between the corresponding terminals. The metal path in turn is attained by a number of metal layers (130, 140, 160, 170 and 180) and vias (135, 145, 165, 175), as is well known in the relevant arts. While the metal layers provide a conductive path, vias provide connectivity from one metal layer to another metal layer at a desired location (according to the desired circuit design). Accordingly, it may be appreciated that an integrated circuit generally contains several different layers.
Masks are used to fabricate many integrated circuits of the same circuit design, as is also well known in the relevant arts. In general, a mask is used to control the manner in which specific areas of each layer are treated (e.g., where a metal is laid or where the metal is etched) in the fabrication steps. The masks are designed to lay the layers, which together implements the circuit elements and the desired connectivity. The masks thus designed are used for fabricating the integrated circuits.
There is often a need to change connectivity between elements of a circuit design. The need for such a change can be due to reasons such as improved performance, faults in previous circuit etc. For illustration, it is assumed that FIG. 2A represents an existing (portion of a) circuit connectivity according to a design specification and FIG. 2B represents the desired changed circuit.
Both circuits are shown containing resistors 230, 240 and 260 and terminals of the resistors are respectively identified as A and B, C and D, and E and F. In FIG. 2A, the circuit connectivity indicates that the terminals A, C and E are connected together and terminals B, D, and F are connected together. On the other hand, in FIG. 2B, terminals A and F are connected together, terminals B, C and E are connected together, and terminal D will be connected to the rest of the circuit (not shown in the Figures.).
Changes to connectivity requires redesign of at least some masks, as illustrated with reference to FIGS. 3A and 3B. FIG. 3A represents an example layout corresponding to the integrated circuit of FIG. 2A. Integrated circuit is shown containing base layer 310, via interface layer 320, metal layers 330, 340, 360, 370 and 380 and via layers 335, 345, 365, and 375.
Base layer 310 implements resistors 130, 140, 160 and is shown by respective terminals. Via interface layer 320 extends the terminals A-F to external metal layer for connectivity. Metal layer 330 makes contact with via interface layer 320. No circuit connectivity is provided on metal layer 330 due to various design constraints (e.g., signal interference conditions) posed by other portions (not shown) of the IC. As a result, terminals A-F are further extended to metal layer 340 by vias on layer 335.
Metal layer 340 provides connectivity between terminals A and C (portion 341), and D and F (portion 342). Due to design constraints or barrier, it is assumed that the remaining connections cannot be provided on this layer. As a result, terminals C, B, E, and D are further extended to metal layer 360 using vias in layer 345. cannot be provided on this layer. As a result, terminals C, B, E, and D are further extended to metal layer 360 using vias in layer 345.
Metal layer 360 provides connectivity (portion 361) between terminals C and E only, and remaining connection between terminals B and D is provided on metal layer 380 (portion 382). Hence, terminals B and D are extended to metal layer 380 using vias on layer 365 and 375 and a metal layer 370.
Thus, masks (9 in addition to those for base layer 310) are generated for each of metal and via layers, and the masks are then used to fabricate ICs as noted above. To illustrate the number of masks that may be needed to be changed corresponding to the circuit design change from FIG. 2A to 2B, the layout corresponding to FIG. 2B is described below with respect to FIG. 3B.
FIG. 3B represents the layout corresponding to the integrated circuit of FIG. 2B. Since changes are reflected only in connectivity of the circuit, implementation of base layer 310, via interface layer 320, metal layer 330 and via layer 335 remain unchanged. For conciseness and clarity, the changes in each layer are described in relation to FIG. 3A.
A change is incorporated in metal layer 340 by removing metal portions (represented by 341 and 342 in FIG. 3A) making connection between A and C, and D and F respectively. These changes are necessitated based on the differences of FIG. 2A and FIG. 2B.
Continuing with respect to FIG. 3B, on metal layer 360, connectivity between terminals C, B, and E is implemented (portion 366) (as shown necessary in FIG. 2B). On metal layer 380, connectivity between terminal B and D is removed (portion 382) and connectivity between terminals A and F is provided. As a result, terminal A and terminal F, which were available only up to metal layer 340 in FIG. 3A, are now shown extended up to metal layer 380 using vias/metals on layers 345, 360, 365, 370, 375 and 380. In addition, a new metal path (portion 386) connecting terminal A and F is laid.
As a result, a total of seven masks representing metal layers 340, 360, 370 and 380, and masks representing vias on layers 345, 365, and 375 are re-generated (redesigned) to incorporate changes in circuit connectivity from FIG. 2A to FIG. 2B.
In general, it is desirable to minimize the number of masks (at least to reduce the overall costs associated with the change) which would need to be changed, when making such circuit connectivity changes.
In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.